Process and device for determining the refractive index of different mediums

ABSTRACT

The proposed fibre-optic sensor is based on surface plasmon resonance and consists of a monomode fibre with multiple angled facets at its end face and suitable coatings. A first partial face is preferably metal-coated, a second is provided with a conventional detection layer, and the angle between the two partial surface is 30°, 45° or 90°.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a Process and to a device for carrying out theProcess, by means of which the refractive index of various media can bemeasured. For this purpose there is utilised the excitation of surfaceplasmons in a boundary layer metallic dielectric and the closedependence of the resonance conditions on the refractive index of thelayer or of the coating system adjoining the metallic layer. Theinvention relates in particular to the emission and derivation ofsignals by fibre optics. Possible applications of the invention lie inthe construction of fibre-optic sensors for measuring physical, chemicaland biological magnitudes, which may be indirectly detected through analteration in refractive index.

2. Description of Background Information

Fibre optic SPR sensors have been intensively investigated for someyears. The first attempts (EP-PS 0 410 505) proceeded in this respectfrom the conventional Kretschmann arrangement with a solid glass prism,to multi-mode fibres being used for coupling and decoupling themonochromatic light. The intensity of the radiation reflected at thereceiving fibre is measured, the operating point being set bydisplacement of this fibre.

A sensor is known from the publication "Fibre-optic sensor based onsurface plasmon interrogation", Letizia De Maria, et. al. Sensors anActuators B, 12 (1993) 221-223, which uses the obliquely-ground andcoated end face of a mono-mode fibre as a sensor head.

Here also operation was with monochromatic light from a He-Ne-laser andthe polarisation was set by means of a rotating λ/2 delay plate. Thesinusoidal signal arising at the detector has a minimum and a maximum,as the surface plasmons are only excited by the TM-wave, whereas theTE-wave serves as a reference signal. Precise measurement in thisarrangement requires optimum polarisation and a high degree of stabilityof the wavelength. A disadvantage in this case is the necessarymechanical system for rotation of the λ/2 plate and the low intensitydiffused back by the sensor head.

A further SPR-sensor with a glass fibre is known from EP-PS 0 326 291,in which however the fibre-optic derivation of the signal obtained isnot possible.

A fibre-optic SPR-sensor proposed in DE-PS 43 05 830 is adjustable onlywith difficulty due to the use of two fibres for exciting andretroreflection of the coupled light.

A further SPR-sensor on the basis of monomode fibres is described in New"in-line" optical-fibre sensor based on surface plasmon excitation, R.Alonso, F. Villuendas, J. Tornos and J. Pelayo, Sensors and Actuators A,37-38 (1993) 187-192. In this case excitation of the surface plasmonresonance is effected with a thin gold layer (15-35 nm) on the groundperiphery of a monomode fibre embedded in an epoxy resin.

Here also linearly polarised, monochromatic light is coupled into thefibre and the transmission is evaluated as a measurement magnitude.

Due to the use of coherent sources and the necessary stabilisation ofwave length, good-value sensors are difficult to manufacture in thisvariant.

In the publication A Novel Surface Plasmon Resonance based Fiber OpticSensor Applied to Biochemical Sensing, R. C. Jorgenson et. al., SPIEVOL. 1886 (1993), p. 35-48, the vapour-coated cylinder surface of thecore of a multi-mode glass fibre serves to excite surface plasmons withbroad-band light. The signal reflected at the point of the fibre isanalysed with the aid of a spectrometer. The wavelength of the dampingmaximum is in this case a measure for the refractive index of the mediumadjoining the metallic layer. A serious disadvantage of this arrangementresides in the intense propagation of the resonance due to the number ofmodes, and thus the multiple excitation of surface plasmons at differentwavelengths.

The object of the invention is to propose a fibre-optic SPR-sensor whichis favourable in terms of energy and which is suitable for use withmulti-mode fibres, without accepting the disadvantageous propagation ofthe resonance due to a plurality of different modes.

This object is achieved according to the invention by a process asdefined in claim 1. The device according to the invention is thesubject-matter of claim 2.

Further preferred developments of the invention are the subject-matterof the sub-claims.

The process according to the invention for exciting surface plasmonresonance is advantageously suitable for emission and derivation ofsignals by means of optical fibres. In particular when multi-mode fibresare used along with broad-band light, good-value sensors may potentiallybe produced. In this respect progress in the development of miniaturisedspectrometers, for example in LIGA technology, was considered. Incomparison to vapour coating of the cylinder surface, by means of thearrangement due to the invention a narrower resonance was achieved dueto the possible radiation collimation through corresponding lenses. Itis possible, in the further developments of the arrangement with planarterminal surface portions, by using a polarisation filter, to excludethe TE-wave, so that the intensity of reflected light of the resonantwavelength can come to 0%.

A particular advantage of variants which operate without mono-modefibres or coherent sources, resides in good-value components and lowsensitivities to fluctuations in wave length.

Further advantages, features and possible applications of the presentinvention will become apparent from the following description inconjunction with the drawings, which show:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the end surface of an optically transparent body with twoplanar, oblique partial surfaces,

FIG. 1A depicts an end view of the end surface of FIG. 1.

FIG. 2 depicts the end surface of an optically transparent body withconically-shaped end surface.

FIG. 2A depicts an end view of the end surface of FIG. 2.

FIG. 3 depicts the end surface of an optically transparent body with atruncated-conical end surface.

FIG. 3A depicts an end view of the end surface of FIG. 3.

FIG. 4 depicts circuit diagram of the triggering and evaluation unitgiven by way of example,

FIG. 5 depicts a diagrammatic view of the sensor head.

DETAILED DESCRIPTION OF THE DRAWINGS

As can be seen from FIGS. 1 and 5, light 5 impinging parallel to theaxis of an optically transparent cylinder 1' is reflected from aplurality of partial surfaces 3, 4 of the machined end surface 2 in sucha way that, after at least two reflections, it is returned againparallel to the axis in cylinder 1, surface plasmons being excited byangle and wavelength used in at least one reflection. First partialsurface 3 is inclined at an angle a with respect to the longitudinalaxis of cylinder 1 and includes an SPR-exciting layer or coating 6.Second partial surface 4 is inclined at an angle β with respect to thelongitudinal axis of cylinder 1. The second partial surface 4 servingfor reflection can, if this is of advantage in manufacturing terms, becoated with the same coating system 7, if the SPR-metal layer has asufficiently high reflection coefficient. As shown in FIGS. 1 and 5,angles α+β+90°.

The cylindrical body 1 can in this respect for example be a glass rod, abar lens, a GRIN lens or also an optical fibre. The material used mustbe transparent for the wavelength used.

FIG. 2 depicts the machined end surface 2 as having a conicalconfiguration, and where one side of the conical surface (or partialsurface) is inclined at an angle α with respect to the longitudinal axisof cylinder 5, and the opposite side surface (or partial surface) isinclined at an angle β to the longitudinal axis. In this instance it canbe seen that α=β=30°. FIG. 3 depicts the machined end surface 2 ashaving a frustoconical configuration, and where the angles α and β ofthe opposite side surfaces (partial surfaces) have the same relationshipas those of FIG. 2.

In the example according to FIG. 5 the end surface 2 of a bar lens 8 isso machined that two planar partial surfaces 3, 4 results, which enclosean angle α=β=90°; the angles α and β may be equal to 45°, although thisembodiment is not depicted in the drawings. Thus the intersectingstraight line of the partial surfaces 3, 4 lies vertically to thecylinder axis 1.

At the other end there is located in the focal point of the convexlenses 9 at the other end of the cylinder 1 in the extension of thecylinder axis, the end surface of a multi-mode fibre 10, which serves toemit and derive broad-band light 5.

FIG. 4 shows in diagrammatic form a possible optoelectronic system forevaluating the sensor signal.

The supply lines to source 11, spectrometer 12, 13 and sensor head 14via a plug-in connection 15 are connected to an X-coupler 16. Thespectrometers 12, 13 serve to obtain the sensor signal and a referencesignal for calculating the source spectrum.

We claim:
 1. A process for determining the refractive index of differentmediums, in which chemical, physical and biological magnitudes aredetectable by surface plasmon resonance (SPR) by using an optical beamcoupled into a cylindrical body having a longitudinal axis andtransparent to the optical beam, comprising:passing said coupled opticalbeam into the cylindrical body; reflecting the coupled optical beam, byat least two reflections, from an end surface of the cylindrical body toa coupling and de-coupling point, via at least two partial surfacesinclined toward one another at the end surface, the at least two partialsurfaces being inclined at an angle with respect to the longitudinalaxis of the cylindrical body; and, exciting surface plasmon resonance atthe end surface by said optical beam, said surface plasmon resonancebeing excited at least during one such reflection, at a wavelength ofthe optical beam.
 2. A device for use in determining the refractiveindex of different mediums by surface plasmon resonance (SPR),comprising:an optically transparent cylindrical body having alongitudinal axis and an end surface forming at least two partialsurfaces, said at least two partial surfaces being inclined at an anglewith respect to the longitudinal axis of said cylindrical body, andwherein said at least two partial surfaces includes a first partialsurface inclined at an angle (α) towards the longitudinal axis of thecylindrical body; and, at least one of said at least two partialsurfaces having an SPR-exciting layer or layer system formed thereon. 3.Device according to claim 2, wherein said at least two partial surfacesinclude a second partial surface inclined at an angle (β) with respectto the longitudinal axis of the cylindrical body, and each said partialsurface being selected from one of a reflective surface and a surfaceprovided with a coating exciting surface plasmon resonance.
 4. Deviceaccording to claim 3, characterised in that the sum of the angles α andβ comes to 90°.
 5. Device according to claim 3, wherein said at leasttwo partial surfaces are formed by portions of one of a conical and afrustoconical end surface of said cylindrical body.
 6. Device accordingto claim 5, wherein the angles α and β are equal to 30°.
 7. Deviceaccording to claim 3, wherein the angles α and β are equal to 45°. 8.Device according to claim 2, wherein said cylindrical body is acylindrical light-wave guide in the form of an optical fiber or a barlens.
 9. Device according to claim 2, wherein the optical beam may becoupled and decoupled via one of an external optical lens, a lensintegrated in the cylindrical body and an optical system.
 10. Deviceaccording to claim 2, further comprising a polarization filter toexclude the TE-wave component.